Tethering control

The Tether Manager listens for commands on the /pps/services/tethering/control object.
If we want to observe responses from the tetherman service, we need to force the shell to keep the file descriptor
open (because this is a server object).
In the following example, we use the exec command to do this, then direct the response messages to
stderr. After this, we send a start command to start a tethering session: